Strategic Progress in Foam Stabilisation Towards High-Performance Foam Concrete For Building Sustainability - A State-Of-The-Art Review

Journal of Cleaner Production 375 (2022) 133939
Contents lists available at ScienceDirect
Journal of Cleaner Production

journal homepage: www.elsevier.com/locate/jclepro
Review
Strategic progress in foam stabilisation towards high-performance foam

concrete for building sustainability: A state-of-the-art review
Nghia P. Tran a, Tuan N. Nguyen a, **, Tuan D. Ngo a, *, Phung K. Le b, c, Tuan A. Le c, d
a
Department of Infrastructure Engineering, The University of Melbourne, VIC, 3010, Australia
b
Faculty of Chemical Engineering, Ho Chi Minh City University of Technology (HCMUT), Viet Nam
c
Vietnam National University Ho Chi Minh City, Linh Trung Ward, Thu Duc District, Ho Chi Minh City, Viet Nam
d
Faculty of Civil Engineering, Ho Chi Minh City University of Technology (HCMUT), Viet Nam
A R T I C L E I N F O A B S T R A C T
Handling Editor: Jian Zuo Lightweight, high energy reservation and excellent functional performance are the main benefits of cellular
concrete. Its properties mainly depends on the pore characteristics, governed by different factors. With an
Keywords: attempt to further expand the current body of knowledge in the field of lightweight concrete, this review paper
Lightweight cellular concrete presents state-of-the-art techniques in foam stabilisation for maximising the performance of foam concrete. The
Foam concrete
kinetics of foam degradation, current progress, trending applications in building and construction sectors, as well
Alkali-activated binders
as potential challenges encountered in developing foam concrete were also elaborated. Overall, the current state
Foam stabilisation
Surfactant of strategies for high foam stability involves the optimal selection of binder combination, aggregate substitution,
Thickening agent internal reinforcement, curing regime, treatment methods, gas-liquid interface modified by nanomaterials,
Curing regime polymer chains and surfactant monomers. The breakthrough in current 3D printable foam concrete technology
Surface treatment and the main challenges with mixing protocols, long-term durability, technological development, energy effi
Fibre reinforcement ciency and economic feasibility were discussed. These provide a holistic approach for future research and
3DP foam Concrete development in manufacturing novel foam concrete.
Sustainability
1. Background and current progress were applied for highway, bridge, tunnel works and low-rise structures
in the US and Europe (Brady et al., 2001). Its success and impressive
By dint of intensive exploitation and high embodied energy con performance inspired the widespread use of foam concrete for building
sumption, the building and construction industry holds accountable for applications (e.g. sandwich composites, wall panels, facades, blocks, and
approx. 38% of global CO2 emissions and 35% of global energy con floor slabs). The main driver for the growing popularity of foam concrete
sumption (United Nations Environment Programme (UNEP), 2020). In in recent years has also been attributed to its suitability for sustainable
this context, building energy efficiency and sustainability in construc designs of modern buildings, such as Eco-sustainability (i.e. CO2 emission
tion has gained extensive attention worldwide in a bid to alleviate mitigation, energy reservation), Economic savings (i.e. minimisation of
global-warming emissions. Accordingly, foam concrete has been dead loads for structure, less labour-intensive operation and limitation
emphasised as a promising energy-efficient building material and a of thermal losses), and Comfort of living (i.e. ideal indoor temperature,
potential avenue for incorporating recycled waste products towards a noise reduction) (Ricciotti et al., 2020). Reportedly,
sustainable future of construction with a low carbon footprint (Shang lightweight-foam-concrete-embedded exterior walls are able to
et al., 2020; Shah et al., 2021a). Historically, foam concrete technology conserve the consumed energy of up to 50% of the total heat power
was first patented by Bayer and Erikkson dating back to the early 1920s supplied in the building (Zhang et al., 2021a). In particular, external
(Valore, 1961). Its applications, nonetheless, were not globally recog walls made of lightweight cellular concrete for residential houses with a
nised until the late 1980s, when the full-scale trials of foam concrete density of less than 1100 kg/m3 have been put into practice in Germany,
* Corresponding author.
** Corresponding author.
E-mail addresses: nghia.tran1@unimelb.edu.au (N.P. Tran), tuan.nguyen@unimelb.edu.au (T.N. Nguyen), dtngo@unimelb.edu.au (T.D. Ngo), phungle@hcmut.
edu.vn (P.K. Le), latuan@hcmut.edu.vn (T.A. Le).
https://doi.org/10.1016/j.jclepro.2022.133939
Received 10 April 2022; Received in revised form 1 August 2022; Accepted 28 August 2022
Available online 13 September 2022
0959-6526/© 2022 Elsevier Ltd. All rights reserved.
N.P. Tran et al. Journal of Cleaner Production 375 (2022) 133939
Switzerland and Netherlands since 2003 (Fig. 1). Lightweight porous of constituent materials and treatment methods is of a preliminary
building walls and innovative glazings are of great potential for approach to enhance the fundamental properties to yield
low-to-zero energy buildings, especially suiting regions exposed to high-performance foam concrete.
extremely hot/cold weather and solar radiation. To date, considerable research has been conducted and reviewed,
Foam concrete is categorised as lightweight concrete made by establishing a current body of knowledge and a long history of foam
introducing the air void system (minimum of 20% per volume) to a concrete development. During the 2000s, the initial studies basically
slurry or mortar matrix by means of mechanical foaming (i.e. pre- analysed the variation in fundamental and functional properties of
forming and mix foaming) or chemical foaming (i.e. aeration) (Fig. 2) cellular concrete in relation to the adjustment of binder types, foaming
(Samson et al., 2016; Newman and Clarke, 1993). Serving as a thermal types/dosage/method and curing regime (Ramamurthy et al., 2009;
insulator for either non-structural or semi-structural purposes, cellular Narayanan and Ramamurthy, 2000). The foaming techniques and con
concrete normally possesses dry density varying from 300 to 1920 stituent materials, nonetheless, still exhibited many sources and tech
kg/m3, a strength of below 25 MPa and thermal conductivities in a range nology limitations. Notably, the increasing interest in high-performance
of approx. 0.1–1 W/mK (Chaipanich et al., 2015; Newman et al., 2003). foam concrete has evolved since the late 2000s (Just and Middendorf,
In order to consider for (semi-)structural application, the foam concrete 2009). In the period 2010–2018s, the major innovation marking the
often possesses a compressive strength of beyond 17 MPa and a density development of high-performance foam concrete involved yielding
of over 1350 kg/m3, according to ACI 213R-03 (ACI Committee three-phase foams, comprising pozzolanic nanomaterials as solid foam
213R-03, 2003) and Eurocode 2 (EN, 1992, 2004). Foam concrete stabilisers and nucleation seeding for the growth of hydrate phases
mostly exhibits superior functional performance over conventional (Krämer et al., 2015a, 2015b, 2017). A variety of fillers, setting accel
concrete and other insulated composites made of either polymeric or erators and foaming agent types expanded the research approaches to
natural materials (Martínez et al., 2019). However, its applications in the analysis of compatibility and relationship between mixed in
construction fields were statistically less than 5% due to the deficiency gredients toward high-strength foam concrete (Bing et al., 2012; Batool
of concern, material reliability, long-term durability and feasible tech et al., 2018; Qu and Zhao, 2017). Also, the emergence of innovative
nology (Zhao et al., 2015a). In fact, retaining foam stability during the geopolymer foam with newer aluminosilicate-rich precursors began to
fresh state until hardening to produce a stable foam concrete is depen pave the way for widely adopting these advanced porous materials in
dent on several factors, including the selection of materials and foaming the construction market (Zhang et al., 2014; Bai and Colombo, 2018).
agents, mixture design, and technology in the production process Since 2018, the research advancement in materials and processing
(Ramamurthy et al., 2009). The hardened properties of foam concrete techniques has attracted a vast number of studies on strength-enhanced
are sensitive to the pore system; thus, pore characteristics (i.e. foam concrete. This period has witnessed a surge in the quantity of
morphology, size, connectivity) are key aspects. These directly influence technical and review publications in both geopolymer (Dhasin
stress distribution and permeability across the materials, reflecting their drakrishna et al., 2021a; Degefu et al., 2021; Novais et al., 2020; Łach,
ultimate strength and durability. Accordingly, the optimum regulation 2021; Nodehi, 2021; Walbrück et al., 2020) and cementitious foam
Fig. 1. (a) Lightweight concrete (Gartmann house – Schweiz, 2003) with a density of 1100 kg/m3, thermal conductivity of 0.32 W/mK, strength of 12.9 MPa; (b)
Infra lightweight concrete (Schlaich house – Berlin, 2007) with a density of 760 kg/m3, thermal conductivity of 0.18 W/mK, strength of 7.4 MPa; (c) Lightweight
concrete (house H36 – Stuttgart, 2012) with a density of 1000 kg/m3, thermal conductivity of 0.23 W/mK, strength of 10.9 MPa; (d) Infra lightweight concrete
(Pavilion – TU Eindhoven, 2015) with a density of 780 kg/m3, thermal conductivity of 0.13 W/mK, strength of 10 MPa (Adapted from (Elshahawi et al., 2021)).
2
Fig. 2. Classification of lightweight concrete based on types and strength/density.
(Chica and Alzate, 2019; Kalpana and Mohith, 2020; Junaid et al., 2022; regard, this review elaborates on strategic concepts regarding foam
Gencel et al., 2022a). Different types of thickening agents (e.g. xan stabilisation and its benefits in enhancing the properties of foam con
than/welan gum, hydroxypropyl methyl cellulose) (She et al., 2018a), crete. The paper also presents the kinetics of foam degradation, per
composite binders (Lesovik et al., 2020), and fibre reinforcement (e.g. spectives, potential applications and challenges encountered in
polypropylene, carbon, glass) (Amran et al., 2020; Mugahed et al., 2020) developing the innovative foam concrete technology. The term “foam
in the form of individual or hybrid combination were introduced into the concrete” in this study indicates all types of cellular, porous or aerated
foam mixtures for maximising their synergetic effects. Analytical and concrete irrespective of binder types, mechanical foaming or chemical
numerical models have evolved intensively to stimulate and predict the aeration method. The only use of porous aggregates to produce light
mechanical behaviour (Nguyen et al., 2017, 2018, 2019a, 2019b; Ullah weight aggregate concrete (LWAC) falls out of the scope of this review.
et al., 2022) and thermal performance of porous medium (She et al.,
2014, 2018b). A notable breakthrough should be given to the 3D 2. Kinetics of foam destabilisation
printing foam concrete technology (Bedarf et al., 2021) and 3D imaging
using X-ray micro-computed tomography (Pang et al., 2018; Kim et al., Foam is a typical gas-liquid system where the gas bubbles are
2020; Yuan et al., 2021; Hajimohammadi et al., 2017a; Wan et al., randomly dispersed within a liquid medium. Depending on the liquid
2017), which provide new insight into characterising pore morphology fraction (φ), foam is liable to self-organise to minimise the surface en
and bio-inspired designs of foam concrete. ergy, resulting in various pore structures and morphology. At low liquid
To further exploit the potential of foam concrete for revolutionary fraction (φ < 1%) or high gas content, closely-packed bubbles deform
applications, it necessitates a comprehensive review of strategies and each other, and thus its structures take on polyhedral shapes (Fig. 3)
methods to optimise pore structure and overall performance. In this (Langevin, 2017). Thin lamellae (i.e. liquid film) in the polyhedral foam
Fig. 3. Foam types and foam destabilisation mechanisms (Reproduced from (Langevin, 2017; Pott et al., 2008)).
3
render its system – especially the top surface, fragile and sensitive to the 3. Strategic regulation for foam stabilisation
temperature gradient, shock and vibration. Spherical bubbles are
observed in regions with a high liquid medium. Thick lamellae between 3.1. Optimal selection of binder combination
spherical bubbles, nonetheless, is easily subjected to gravitational
drainage. In the case of foam slurry/concrete with a density beyond 400 The control of yield stress evolution, plastic viscosity, setting/gelling
kg/m3, the pore structure is mainly featured by spherical shapes (Dha time and consistency of matrix slurries in the fresh state play a critical
sindrakrishna et al., 2020; Jones et al., 2016). role in ensuring the stability of air bubbles, reflecting the hardened
Due to the high gas-liquid interfacial area, foams are prone to be properties of foam concrete (Ahmed et al., 2009; Dhasindrakrishna
thermodynamically unstable and can collapse within a short period of et al., 2021c). Cement fineness and types can thus determine the degree
time after gas incorporation. Foam degradation involves a complex of stabilisation of foam mix at low densities. Apart from ordinary port
process with three main underlying mechanisms overlapping over land cement (OPC), several typical binders are widely used for reducing
ageing (see Fig. 3). These foam kinetics stems from a combination of (i) setting time and improving the early-age strength of foam concrete.
hydrodynamic drainage – a phenomenon that refers to the liquid suction These include rapid-hardening cement (Kearsley and Wainwright,
at lamellae and Plateau borders driven by the gravity, capillary pressure 2001a), calcium sulphoaluminate (CSA) cement (Liu et al., 2019a,
and viscous stress, which accelerates a thinning rate of bubble film and 2021a), high alumina cement (Huang et al., 2005; Chen et al., 2014;
gas-liquid segregation (ii) coalescence (i.e. lamellae rupture), – a merge Chen and Liu, 2013), phosphogypsum cement (Cui et al., 2020; Bumanis
of adjacent bubbles into a larger one due to a rupture of thin lamellae et al., 2020), magnesium cement (Yue and Bing, 2015; Fu et al., 2016;
caused by high surface stress, and (iii) inter-bubble gas diffusion (i.e. Ma and Chen, 2017; Hao and Li, 2021) and alkali-activated binders (Hao
Ostwald ripening, coarsening or disproportionation) – a diffusion of gas et al., 2022; Zhang et al., 2015; Hajimohammadi et al., 2017b; Roviello
from small to large bubbles, due to a discrepancy in Laplace pressure, et al., 2017). The replacement ratio could be employed in a range of
leads to the growth of large bubbles at the expense of tiny bubbles 25–100% of the binder content (Amran et al., 2015). A high content of
(Fameau and Salonen, 2014; Walstra et al., 1989; Cantat et al., 2013; CSA tends to decrease the zeta potential of the blends, which indicates a
Weaire and Hutzler, 2001). Ostwald ripening is considered as the weak repulsive electrostatic force and a correspondingly strong van der
leading foam destabilisation mechanism, whereas drainage is observed Waals attraction among cement particles (Huang et al., 2019a). Hence,
to take a dominant role over the ripening effect in the case of the low this causes high dynamic yield stress and viscosity of the paste mix to
yield stress of media (Feneuil et al., 2019a). These three mechanisms increase the strength of liquid film during the fresh state. The CSA
promote the expansion of bubbles over increasing time until a complete content of 25% in the blended binders was found to be the optimum
collapse (Zhou et al., 2020). A mutual acceleration between these ki dosage to achieve both high early- and late-age compressive strength
netics even triggers a rapid foam collapse within a few second, especially (Huang et al., 2019a). Higher early-age strength development of foam
in a case of low foam liquid fraction. concrete is attributed to the rapid formation of cement hydrates such as
The properties of fresh foam base mix strongly reflect the post- ettringite which strengthens the pore structure before foam degradation
hardening pore morphology and structure, which in turn modify the (Ge et al., 2020). Magnesium phosphate foam concrete can obtain 80%
properties of concrete such as strength, shrinkage, thermal conductivity, of 28-day strength at 7 days with a narrow pore size distribution,
water permeability and sound absorption (Dhasindrakrishna et al., thereby achieving higher strength than the OPC foam concrete (Li et al.,
2021b; Gu et al., 2020). To date, foam stability is dependent on the 2019a). In the alkali-activated cement system, the Na-based activators
characteristics of a three-phase system, including the gas phase (i.e. and precursors with high specific surface areas (SSA) usually induce
solubility in water, gas content), liquid phase (i.e. rheology and density higher plastic viscosity, viscoelastic evolution and yield stress of the
of base mix, setting time, ionic bonding of foaming agent) and solid paste mix than those with K-based activators and additives with low SSA
phase (i.e. fineness, surface charge and morphology of particles). Gov (Lu et al., 2021a).
erning these aspects to ensure the stability of generated foam with thick It should be noted that the state of balance among confinement force
lamellae and fine bubble texture (e.g. small spherical bubbles, uniform (Fc), internal pressure (Pi), drainage force (Fd), buoyancy force (Fb), and
size distribution, closed cellular microstructure) can yield an ideal surface tension (Fst) should be reached to attain a statically stable air
porous system. An uniform and closed cellular microstructure of hard bubble (Fig. 4a) (Li et al., 2021). Under the mixing process, foam is
ened foam concrete are indicative of superior engineering performance subjected to the frictional force (Ff), which is reflected through the
(Jones et al., 2016). To do so, sufficient viscosity and yield stress, rheological properties of the foam slurry (Li et al., 2021). Quick-setting
together with low surface tension, should be taken into serious consid time and high viscosity of foam mixture can avert the foam destabili
eration as crucial factors behind the foam stabilisation (Pott et al., sation and settlement when being subjected to vibration or disturbance
2008). Notably, the high viscosity of mixtures without hindering the (Feneuil et al., 2019b; Zeng et al., 2020). Nonetheless, these rheology
dispersion of gas bubbles facilitates the retarding of bubble rearrange characteristics of the interstitial paste should maintain at an optimum
ment, drainage rate and bubble size growth (Feneuil et al., 2019b). A range to avert both foam collapse and heterogeneous pore distribution
high rate of yield stress evolution by using rapid-hardening cement can corresponding to the extremely low or high viscosity of foam mixture
contribute to the early establishment of a firm skeleton surrounding gas (Dhasindrakrishna et al., 2021c).
bubbles, counterbalancing against the destabilising stress (Feneuil et al., Additives are often utilised for modifying the fresh characteristics of
2019a). Furthermore, the ionic charge of a foaming agent can influence the paste foam mix, which are derived from manifold sources. These
foam stability due to the interaction between anionic surfactant mole include fly ash (FA) (Chen et al., 2021; Hanif et al., 2017; Chindaprasirt
cules and cations of binders (e.g. Ca2+, Al3+) (Sahu et al., 2018). The and Rattanasak, 2011; Sai Krishna et al., 2021; Batool and Bind
strong absorption of nanoparticles at the gas-liquid interface caused by iganavile, 2018; Kearsley and Wainwright, 2001b), bottom ash (BA)
electrostatic attraction has been found to avert liquid drainage, film (Gencel et al., 2021a), palm oi fuel ash (POFA) (Jhatial et al., 2021;
rupture and gas diffusion. The addition of different fillers and fibres for Abraham et al., 2021; Liu et al., 2016), ground granulated blast furnace
internal restraining and pore refining effect exhibits certain effective slag (GGBFS) (Jose et al., 2021; Pan et al., 2007), steel slag (Zhou et al.,
ness in limiting drainage and adjacent bubble interaction. These afore 2021; Gong and Zhang, 2019; Park and Choi, 2021), furnace slag (Song
mentioned approaches for high foam stability necessitate a regulation in et al., 2021a), silicafume (SF) (Batool and Bindiganavile, 2020a; Ahmad
constituent materials on purpose, which are elucidated in the following and Chen, 2019; Chung et al., 2020; Zhang et al., 2020a), limestone
section. (Cong and Bing, 2015; Yang et al., 2021), lime mud (Yuan et al., 2022),
metakaolin (MK) (Jiang et al., 2016a; Sepulcre Aguilar et al., 2013;
Sarazin et al., 2021), sepiolite (Pinilla Melo et al., 2014), biochar (Gupta
4
Fig. 4. Schematic diagram of (a) forces acting on a single air bubble, and (b) the foam slurry incorporating fly ash (Adapted from (Li et al., 2021)).
et al., 2022; Falliano et al., 2020a), sugarcane bagasse ash (SBA) (Kha microstructure, thicken pore wall and refine pore structure, which re
waja et al., 2021), incinerated sugarcane filter cake (ISF) (Makul and sults in an enhancement in performance of foam concrete.
Sua-iam, 2016), aerogel powder (Li et al., 2019b), bentonite slurry (Xie Another important factor for the stable foaming process is the setting
et al., 2018), dredged river sediment (Yang et al., 2020a), eggshell speed of foam slurry by using a small number of hardening accelerators
powder (Tiong et al., 2020), hollow cenosphere (Zhang et al., 2020b; in foam mixtures, which quickly promote the initial structural formation
Hajimohammadi et al., 2019), recycled glass (Kashani et al., 2019; Khan to stabilise porous slurry in a short period of time (Pan et al., 2014;
et al., 2019), superabsorbent polymer (SAP) (Yuanliang et al., 2022), Krauss Juillerat et al., 2010; De Windt et al., 2015). Tian et al. (2016)
ethylene vinyl acetate (EVA) powder (Sang et al., 2016), and sand-based reported that the presence of accelerators triggered a reduction of initial
breathing brick (Wang et al., 2018). The influence of different mineral setting time from 476 min to 70–102 min (approx. 78.6–85.3%) and a
additives on the foam stability varies on its fineness, particle surface decrease of final setting time by 22.9–46.5%. Such acceleration of hy
properties, chemical composition, amorphous content and types of ions dration averts the foam degradation over increasing time. Pore refine
released into water (Huang et al., 2019b). In general, approx. 30–70% of ment effect is pronounced with increasing set-accelerator dosage to 3–5
FA, 10–50% of GGBFS, and up to 10% of SF have been introduced in the wt%, but no further effects thereafter (Jiang et al., 2016b). Also, ac
foam mix as cement replacement to enhance the consistency of the mix celerators can promote nucleation seeding effects and the solubility of
and provide long-term strength. Smooth and spherical surfaces of FA are free lime from cement to facilitate the hydration of clinker minerals and
prone to decrease the frictional force of the paste and thus reduce the precipitation (Dorn et al., 2022; Steshenko et al., 2020), which in turn
impact on foam stability under mixing (Fig. 4b) (Li et al., 2021). The significantly improves early and later strength of foam concrete up to
ball-bearing effect of FA also acts as a hindrance limiting the contact 50% (Myrdal, 2007). Adjusting setting time by accelerators reduces the
areas among air-liquid interfaces, thereby narrowing the pore size and plastic shrinkage of foam slurry by 47% and 65%, corresponding to the
improving strength accordingly. Higher fineness of FA yield more closed use of 2% calcium chloride and 0.5% calcium oxalate accelerators,
void and thick pore wall than the coarser FA. For GGBFS, due to respectively (Steshenko et al., 2020). However, a dosage of hardening
CaO-rich source and angular shape, its presence in the mix facilitates accelerators should be appropriately selected to reach an equivalent pH
particle-particle interactions and early-age gel formation (Mohammed of the foaming agent (Siva et al., 2017), while it can prevent the
Fouad et al., 2020). This results in an increase in the viscosity and yield retarding effect at low content and the impact on long-term strength at
stress over time (Mohammed Fouad et al., 2020), which correlates to an high dosage (Myrdal, 2007). For example, Siva et al. (2017) pointed out
improvement in foam stability and strength. On the other hand, fine FA that the incompatibility between conventional accelerators (e.g. calcium
and SF tend to remain in the thin film and plateau border. It hinders the chloride, calcium nitrate, triethanolamine, with pH > 6) and natural
drainage of the film liquid and refines pore structure with more uniform soapnut foaming agent (low pH < 4.5) might lead to foam collapse.
and closed circular pores, thereby improving thermal insulation and When an aluminium sulfate accelerator (pH ~ 4.1) was used, it facili
water resistance (Cong and Bing, 2015). High reactivity of these poz tated the setting and showed no effect on foam collapse. Sathya Nar
zolans promote the pozzolanic reaction with Ca(OH)2 to produce sec ayanan and Ramamurthy (Sathya Narayanan and Ramamurthy, 2012)
ondary C–S–H gel and thus improve the strength and shrinkage compared different hardening and set accelerators, namely calcium
resistance (Zhang et al., 2018a; Gökçe et al., 2019). A similar tendency is chloride, calcium nitrate, triethanol amine, and alum. The author also
reported by the addition of other supplementary cementitious materials stated that the alum accelerator exhibited the highest compatibility with
(SCMs) as cement substitutes such as metakaolin, biochar, marble anionic sodium lauryl sulfate foaming agent than other accelerators with
powder, in foam concrete. The use of MK as cement substitution reduce a short setting time. Notably, the use of calcium chloride accelerator
the workability of the foam slurry but yield a denser microstructure and resulted in instability of foam slurry; meanwhile, triethanol amine and
thick pore wall (Batool and Bindiganavile, 2020a; Jiang et al., 2016a). calcium nitrate showed better stable foam in the mixture (Sathya Nar
Quick lime can also facilitate the formation of Ca(OH)2 to perform ayanan and Ramamurthy, 2012).
pozzolanic reaction for strength improvement (Tian et al., 2016).
Harmful acidic impurities and insoluble salts yielded from quick lime, 3.2. Sand replacement by fillers
nonetheless, limit its use in foam concrete to a certain dosage of less than
4% to maintain a homogeneous pore structure (Cong and Bing, 2015; Literature showed the utilisation of various forms of fillers as angular
Tian et al., 2016). It should be noted that the combination of SCMs or sand substitutes. These alternatives to fine aggregate in foam concrete
composite binders can provide synergetic effects to densify comprise FA (Nambiar and Ramamurthy, 2006a, 2007a; Jones and
5
McCarthy, 2005a; Kunhanandan Nambiar and Ramamurthy, 2008), foam mix with a density of 1300 kg/m3 and 90% for those with a density
POFA (Al-Shwaiter and Awang, 2020, 2021; Al-Shwaiter et al., 2022; of 1600 kg/m3. It should be noted that very fine quarry dust may require
Lim et al., 2013), coal bottom ash (CBA) (Kurama et al., 2009; Yang excessive water demand, which in turn result in a larger pore structure,
et al., 2019, 2020b), rice husk ash (RHA) (Kunchariyakun et al., 2015; higher pore interconnectivity, and lower strength (Krishnan and Anand,
Hadipramana et al., 2014), bagasse ash (BA) (Kunchariyakun et al., 2018). In general, the utilisation of quarry waste in foam mix contribute
2018), palm oil clinker (Johnson Alengaram et al., 2013), quarry stone to a lower energy consumption (approx. 24%), greenhouse gases emis
dust (Zafar et al., 2020; Wan et al., 2018; Kumar, 2021), limestone slurry sion (7–10%), and cost production (26%) (Kumar, 2021; Lim et al.,
waste (Kumar et al., 2018), lime mud powder (Li et al., 2022), coal 2017).
gangue (Wu et al., 2021), shale ceramsite (Liu et al., 2021b; Wang et al., FA has been a commonly-used filler as sand replacement in foam
2021), aerogel powder (Zhang et al., 2020c), phase change materials concrete. Acting as micro-aggregate, FA particles trigger the pore
(PCM) (Ramakrishnan et al., 2021), expanded perlite (Abd Elrahman refinement and pore filling effect, resulting in finer pores with uniform
et al., 2019; Rozycka and Pichor, 2016), expanded clay (Lu et al., distribution (Nambiar and Ramamurthy, 2007b). Also, a higher shape
2021b), expanded polystyrene (Kan and Demirboğa, 2009; Dissanayake factor (1.6–1.8) representing irregular pores was observed from foam
et al., 2017; Sayadi et al., 2016; Shi et al., 2019), recycled glass (Haji mixtures with fine sand, while those with fine FA showed more spherical
mohammadi et al., 2018a; Gencel et al., 2022b), foundry sand (Jones pores with a lower shape factor (1.1–1.2). The utilisation of FA as
et al., 2012), plastic granules (Ikponmwosa et al., 2017), tyre rubber aggregate replacement requires less foam volume to achieve the same
crumb (Kashani et al., 2017, 2018; Eltayeb et al., 2020; Wang et al., designed density; hence it aids in maintaining the stable air bubbles with
2019a), fine recycled aggregate concrete (FRAC) (Lermen et al., 2019; an uniform size distribution (Nambiar and Ramamurthy, 2006b,
Favaretto et al., 2017; Pasupathy et al., 2021), pulverised ceramics 2007b). Moderated pore connectivity in relation to improved thermal
(Awoyera and Britto, 2020), and clay brick (Aliabdo et al., 2014; Ibra conductivity and freeze-thaw resistance was reported when replacing
him et al., 2013; Hamad et al., 2020). sand with FA particles (Hilal et al., 2015a, 2015b). According to the
Notably, characteristics of aggregate such as the grading, content, study of She et al. (2018c), foam concrete attained twofold higher
size and shape are the crucial factors affecting the pore morphology and strength with the use of 100% coarse FA (davg = 98 μm) as sand sub
performance of foam concrete (Song and Lange, 2019, 2021). The high stitutes. However, the highest strength was recorded at 50% sand
fineness of fillers can trigger preferential adsorption of surface-active replacement level by FA. Beyond this optimum replacement threshold,
agents and increase the yield stress through a colloidal interaction of strength improvement was limited by an inconsistency in microstructure
contacting particle networks (Zhu et al., 2021). Modifying the fluidity of with a high quantity of unhydrated FA. This could be attributed to the
the matrix constrains the movement and coalescence of air bubbles. low pozzolanic activity of coarse FA and inadequate Ca(OH)2 for the
Accordingly, fine filler materials render air void distribution highly pozzolanic reaction. Foam concrete with FA as fine aggregate also
uniform, while coarse aggregate may cause clustering of air bubbles experienced a higher shrinkage rate due to increased capillary pores and
with large irregular voids (Nambiar and Ramamurthy, 2006b). This low restraining effect of fine particles against shrinkage deformation,
indicates that the higher strength of foam concrete strongly correlates to compared to coarse sand grains (Tran et al., 2021a; Nambiar and
finer fillers at a given density (Song and Lange, 2019). In particular, the Ramamurthy, 2009).
sand replacement by quarry waste powder exhibited a denser micro Sand replacement by fine pozzolanic wastes significantly contribute
structure with further refined pore size and less interconnected voids to the enhanced strength properties of foam concrete (Tambe and
(Fig. 5). A proportion of large pore (>300 μm) decreased to 40–120 μm Nemade, 2021). High reactive silica content in these waste materials
when fine sand was substituted by quarry dust (Lim et al., 2017). Fine promotes pozzolanic reaction to attain a denser microstructure with less
limestone waste fillers could also provide the nucleation sites, which interconnected pores. Rum et al. (2017) reported that foam concrete (γd
promoted a dense formation of C–S–H gel and fast growth of AFt phase = 1650 kg/m3) with 40% RHA as sand substitution could achieve 28-day
in the microstructure of foam concrete (Kumar et al., 2018). These ef compressive strength at 22.4 MPa, with an improvement rate of 48.3%.
fects enhanced the strength properties and durability characteristics of In contrast, due to high water demand to maintain workability, the use
foam concrete (Lim et al., 2017; Bagheri and Samea, 2019). In the of CBA as sand replacement is liable to decrease the strength properties
investigation of Bagheri and Samea (2019), complete sand substitution of porous matrix (Li et al., 2018).
by stone powder resulted in a reduction of fluidity and refinement of The strength of foam concrete with lightweight aggregates (e.g.
pore size. By replacing sand with powder, the average pore size distri plastic, tyre crumb, expanded clay, polystyrene) as sand substitutes is
bution (D50) was reduced from 1087 μm to 402 μm. Accordingly, prone to compensate for the improved thermal and acoustic insulation
compressive strength at 28 days was improved by approx. 150% for properties. Strength reduction is mainly attributed to the weak bonding
Fig. 5. Pore structure and distribution for foam concrete (a) without and (b) with quarry dust as sand replacement (Adapted from (Lim et al., 2017)).
6
between cementitious matrices and plastic/rubber. To enhance their cause clustering of air bubbles with interconnected pore and irregular
interface, Kashani et al. (2018) treated rubber crumbs with five diver shapes when employed at high content as sand replacement in foam
gent methods, including cement coating, silica fume coating, NaOH concrete (Favaretto et al., 2017; Aliabdo et al., 2014). However, the low
(10%), KMnO4 (5%) and H2SO4 (10%) soaking. The findings indicated replacement level of sand by these construction and demolition wastes
that all treated tyre crumbs increased the compressive strength of foam (approx. below 20%) shows potential improvement in the performance
concrete by 25–55% due to the improved rubber-matrix bonding. of foam concrete. Especially, coarse-sized FRAC (1.18–4.75 mm)
Interestingly, a significant improvement in strength was reported in the exhibited higher strength improvement for foam concrete than those
case of using fine particles of polyvinyl waste (SiO2 + Fe2O3 + Al2O3 > with medium and fine size (<1.18 mm) due to lower water absorption of
50%) as a sand replacement, which could involve pozzolanic reaction coarse grains (Favaretto et al., 2017). Replacing sand with 25% of clay
(Ikponmwosa et al., 2017). With an augmentation of sand replacement brick showed no significant effect on compressive strength but
level by polyvinyl waste, the density tended to increase and eventually, contributed to an improvement in splitting tensile strength and sound
compressive strength was twice that of the control. Also, using recycled attenuation coefficient (Aliabdo et al., 2014; Ibrahim et al., 2013).
glass as sand replacement exhibited higher strength properties of foam
mix, especially at a low density (Hajimohammadi et al., 2018a; Chandni
3.3. Modification of the gas-liquid interfacial structure
and Anand, 2018). This could be because the foam mixture containing
recycled glass necessitated fewer air bubbles than those with river sand
Stable foam mix has been achieved by using various stabilisers
to achieve the same density (Hajimohammadi et al., 2018a).
ranging from surfactant molecules (i.e. monomer), proteins or polymers
High water absorption of FRAC and clay brick aggregate tend to
to solid nanoparticles. Depending on the concentration or characteristics
Fig. 6. Model of components in cementitious matrix influencing on foaming process and foam stability (Reproduced from (Pott et al., 2008)).
7
of the liquid medium (e.g. salinity, pH), these surfactants and particles effect among ionic surfactant molecules facilitates a stable foamability
self-assemble into a variety of morphologies, such as wormlike micelles, compared to nonionic surfactant molecules (Sahu et al., 2018).
lamellar phases, fractal aggregates or gels (Fameau and Salonen, 2014). The adsorptive behaviour of anionic (sodium dodecyl sulfate, SDS–),
Large-scaled sizes of the stabilisers, such as polymers or nanoparticles, cationic (dodecyltrimethyl-ammonium bromide, DTAB+) and nonionic
have higher adsorption energies with other effects on foam stability. In surfactants (fatty alcohol polyoxyethylene ether-9, AEO-9) on the sur
general, the intertwined-net structures of stabilisers act as a hindrance face of cement particles were compared in the study of Liu et al. (2020).
inside bubble channels to limit the mobility of water molecules and The positive and negative change of zeta potential data (Fig. 7a)
narrow the cross-section of the foam liquid channel (Fig. 6) (Fameau and demonstrated the evidence of the competitive adsorption of cationic and
Salonen, 2014; Lazniewska-Piekarczyk, 2014). On the other hand, the anionic surfactant molecules onto cement particles. It was supported by
presence of aggregated nanoparticles in Plateau borders is liable to in the FTIR (Fig. 7b), which exhibited the change in the intensity for the
crease the fluid’s viscosity against drainage and coalescence. Foam band at 1400 cm− 1 in the case of DTAB + adsorption and at 1240− 1200
stability is more pronounced when combining these stabilisers (see cm− 1 when SDS– is adsorbed via –OSO–3 group (Liu et al., 2020). In
Fig. 6). The roles and effects of stabilisers and components are eluci contrast, a slight change in zeta potential and FTIR was recorded for
dated in the following subsection. nonionic surfactants due to their limited affinity for adsorption on
cement grains. An observation as to the negligible adsorption of
3.3.1. Role of foaming surfactant molecules nonionic surfactant was also reported by several authors (Merlin et al.,
The selection of foaming agents plays a vital role in determining the 2005; Zhang et al., 2001). Regarding the electrical double-layer
stable foam production and bubble foam characteristics since not all adsorptive model (Zhang et al., 2001; Yousuf et al., 1995), the anionic
surfactants act as good foaming agents (Sahu et al., 2018; Ranjani and surfactant can quickly enter the outer layer of adsorbed cement surface
Ramamurthy, 2010; Sun et al., 2018; Samson et al., 2017). Foaming through electrostatic adsorption (Liu et al., 2020), thus cement particles
surfactants developing low surface tension can entrain finer air voids adjacent to the gas-liquid interfaces tend to be drawn onto the bubble
(Behnia et al., 2009). On the basis of origin, foaming agents are classified (Hou et al., 2021). The Ca2+ electrolytes from cement hydration are
into natural (e.g. protein- or plant-based substances containing alkaline prone to diffuse onto the gas-liquid interface, averting the migration of
salts, keratin, or saponin) and synthetic types (Sahu et al., 2018; Kadela SO2−
4 to the air side (Hou et al., 2021). This results in the formation of
et al., 2020). Commonly, plant-based and protein-based foam exhibits shell layers with less ettringite but more C–S–H gels and Ca(OH)2 sur
smaller bubbles with uniform size distribution than synthetic foam, rounding the air pores, whose structure seems to separate from the
which in turn results in higher strength enhancement of foam concrete cement matrix for a certain distance (Hou et al., 2021). Meanwhile, the
(Panesar, 2013; Falliano et al., 2018, 2021; Hashim and Tantray, 2021). slowly-adsorptive behaviour of cationic surfactant molecules in the
The performance of concrete using synthetic foaming agents is depen inner layer may lead to an increase of zeta potential to positive charge
dent on its compatibility with other admixtures in the alkaline envi (Liu et al., 2020, 2021c). In most cases, cement particles hardly absorb
ronment of concrete (Panesar, 2013). Several synthetic surfactants, such onto the bubble foam with cationic surfactant molecules, thereby
as Betain, Tween 20 and Brij 700, have been reported to be incompatible establishing the water-rich areas near the air bubbles during the fresh
with cement solution due to the precipitation after interacting with state (Hou et al., 2021). This triggers the migration of SO2−4 and AlO2
−
calcium or sulfate ions (Feneuil et al., 2017). The distinct properties of ions to the membrane to facilitate the formation and enrichment of
the foaming agent are attributed to the charges of polar groups in hy ettringite in the loose air pore walls (Hou et al., 2021). Notably, the
drophilic heads, indicating either nonionic (neutral), anionic (nega higher and faster degree of surfactant adsorption it takes place, the
tively charged), cationic (positively charged) or zwitterionic smaller pore size foam structure can form (Krämer et al., 2015a).
(amphoteric – both negatively and positively charged) groups (Du and In most cases with respect to a low surfactant concentration,
Folliard, 2005). Due to steric and electrostatic interaction, the repulsive monolayer adsorption on cement surfaces through electrostatic
Fig. 7. (a) Zeta potential and (b) FTIR spectra of cationic, anionic, and nonionic surfactant after adsorption onto cement particles (Adapted from (Liu et al., 2020)).
8
interaction orientates hydrophobic tails of surfactant molecules towards compressive strength to 24.5 MPa – an increment of 236%, as well as
the bulk phase (Du and Folliard, 2005). Hydrophobic cement particles 28-day splitting tensile and flexural strength by 76% and 167%
thus tend to be attracted and set at air-bubble interfaces, thereby respectively. Al-Shwaiter et al. (2021) reported that the use of 1.35%
enhancing foam stabilisation. In addition, inter-particle hydrophobic PCE superplasticiser reduced W/B ratio from 0.6 to 0.4 without
attractive forces at the surface of the cement grains are liable to induce changing the spreadability of the foam mix. The mix with this optimum
concentration-dependent effects on the macroscopic yield stress of the superplasticiser content exhibited high pore wall thickness and narrow
cementitious matrix (Feneuil et al., 2017). It is reported that the increase pore diameters. This was reflected in a significant improvement in
in surfactant concentration correlated to the augmentation of a viscous compressive strength from 12.8 MPa to 20.9 MPa, and flexural strength
surfactant solution which in turn aided in producing greater foam sta by 25.7% at 28 days. Water absorption and gas permeability also
bility and smaller foam bubble size (Sahu and Gandhi, 2021). None decreased by 10.3% and 81.7%, respectively. Huang et al. (2019b)
theless, beyond the critical micelle concentration (CMC), the revealed that the compatibility between polymer-based admixtures and
agglomeration of surfactant molecules into micelles form may act as a foaming surfactant strongly reflected in the foam stability. Anionic
steric hindrance among particles. This phenomenon substantially di surfactants showed better compatibility with PCE with respect to high
minishes the yield stress (Feneuil et al., 2017). Also, these adsorptive foam stability than naphthalene-based superplasticiser. However, the
micelles, with their head oppositely oriented toward the aqueous solu opposite phenomenon was observed when a viscosity-modifying agent
tion, renders the surface of cement grain more hydrophilic. Micelles was combined with a superplasticiser in a foam mix.
have no further surface activity, and thus the foamability start to level Notably, it should be noticed that there is a critical superplasticiser
off at the CMC threshold. Surfactants with a higher CMC threshold, dosage. Beyond this threshold could lead to an anti-foaming behaviour
which depend on the length of the hydrocarbon polymer chain in the and foam collapse (Dhasindrakrishna et al., 2021c; Al-Shwaiter et al.,
hydrophobic tail, are more likely to produce poorer foam stability in the 2021). Due to the polymer-chain adsorption onto the cement surface, a
fresh state (Sahu et al., 2018). In this case, nonionic surfactants often negative surface charge of adsorbed particles provokes the electrostatic
possess a lesser CMC threshold than ionic surfactants due to no elec repulsion to surpass the van der Waals attractive forces among adjacent
trostatic repulsion against micellisation. To optimise the efficiency of cement grains. This phenomenon reduces the dynamic yield stress of the
foam stability, the appropriate selection of surfactant concentration paste and the retardation of cement hydration. Especially, high air-void
below or equal to CMC value is an essential factor. mobility in the mix with high foam content induces the coalescence of
To further achieve stable foamability, several innovative methods air voids, resulting in foam instability over a period of time (Qian et al.,
have also been employed, such as composite/mixed foaming surfactants 2018). This could be seen in Fig. 8a, where the uniformly distributed fine
(Kashani et al., 2020; Bera et al., 2013), Gemini surfactants (i.e. pores were formed with PCE superplasticiser, compared to a heteroge
single-chained molecules bonded with spacer groups) (Qiao et al., 2017; neous pore structure of mix without SP. This effect was prone to
Chen et al., 2017), and microwave and ultrasonic treatment (Kuzielová diminish when increasing superplasticiser dosage in which the coales
et al., 2016). Kashani et al. (2020) pointed out that a composite foaming cence of several adjacent pores formed larger and interconnected coarse
agent mixed between nonionic Triton X-100 and anionic SDS surfactants pores (Pasupathy et al., 2022).
could counterbalance the drawbacks of different foaming agents and Sahu and Gandhi (2021) realised that the use of a maximum of 0.2%
generate highly-stable concrete foam. This resulted in a reduction in the of carboxymethyl cellulose sodium (C8H16NaO8) as anionic polymer
mean pore size by approx. 19% and an improvement of 25% in additives significantly enhanced the viscosity of surfactant solution.
compressive strength compared to the use of Triton X-100 or SDS alone. Large air voids were reduced and led to a 10–30% enhancement in
In general, most of the nonionic and ionic surfactants can be mixed with compressive strength and a 14–25% reduction in water absorption. It
each other on the grounds of compatibility and charge neutralisation. was also noted that the carboxymethyl cellulose sodium is more
However, it should be noted that not all types of foaming agents can be compatible with nonionic surfactants than ionic surfactants owing to the
combined to yield stable foam. For instance, the contrast between absence of charges. Another effective foam stabiliser is calcium stearate
anionic and cationic surfactants renders their combination incompatible (C36H70CaO4) which also exhibits an improvement in strength and
(Sahu et al., 2018). elasticity of bubble shell and provides a nucleation effect to accelerate
the hardening of paste (Cui et al., 2018). Moreover, hydroxypropyl
3.3.2. Role of polymers chains methylcellulose (HPMC) in powder form has been reported as an organic
Polymer-based superplasticisers possess foaming behaviour, which stabiliser that can increase the bubble film and reduce the Plateau
can introduce further air bubbles inside the mix and help maintain border interconnectivity (She et al., 2018a). Notably, HPMC-stabilised
proper rheology for bubble stability when an appropriate dosage is used foam exhibits negligible effects on pore refinement, density and total
(Lazniewska-Piekarczyk, 2014; Lange and Plank, 2012). In the case of porosity, despite an improvement in foam stability (She et al., 2018a;
fixed spreadability, the use of superplasticiser aids in reducing the Zhang et al., 2020b). However, HPMC stabiliser increases the number of
water-to-binder (W/B) ratio, resulting in a uniform air void distribution, closed pores of foam concrete due to the plastic viscosity modification
a reduction in pore connectivity and size, and strength enhancement and increased dynamic yield stress against bubble coalescence, which
(Al-Shwaiter et al., 2021; Wee et al., 2011). With high compatibility contributes to strength improvement and better thermal conductivity
with the foaming agent, the inclusion of polymer-based superplasticiser (Zhang et al., 2020b; Liu et al., 2021d). Sang et al. (2015) pointed out
can further improve the pore structure of foam mixture with smaller that the accumulation of HPMC at the gas-liquid interface reduced the
voids (Jiang et al., 2016b; Hilal et al., 2015a) and high consistency water drainage rate and stabilised the foam bubble in the fresh stage.
(Kunhanandan Nambiar and Ramamurthy, 2008) and distinct Also, such polymer-enriched layers helped control the internal moisture
morphology of pore shell covered by flaky and needle-like crystals for hydration; therefore, shrinkage and cracking were impeded.
(Atahan et al., 2008). Regarding the effect of superplasticiser types, Compared to other thickening agents such as welan gum (WG), HPMC
Chandni and Anand (2018) stated that polycarboxylate ether (PCE) still exhibits higher foam stability (Huang et al., 2019b).
superplasticiser achieved higher efficiency in reducing the In addition, Xanthan gum (XG) – a water-soluble polymer, has also
water-to-binder ratio of the foam mix than sulphonated naphthalene been utilised as a thickening agent to modify the viscosity of foam so
formaldehyde (SNF) superplasticiser. The findings indicated the effec lution and increase the lamellae thickness around the bubbles to limit
tiveness of PCE superplasticiser in decreasing macropores and the exchange of gases, resulting in uniform pore sizes. Hajimohammadi
improving the compressive strength of foam mix by 2.8 times. Likewise, et al. (2018b) reported that foam concrete with XG modifier possesses a
Ling et al. (2018) pointed out that the introduction of 1% PCE super smaller pore size and approx. 34% higher strength properties than those
plasticiser in foam mix (γd = 1500–1600 kg/m3) enhanced 28-day without XG. Likewise, Zhu et al. (2020) observed that XG stabiliser could
9
Fig. 8. Cross-section images of foam concrete modified by (a) superplasticiser and (b) foam stabilising emulsion with different contents (Adapted from (Dhasin
drakrishna et al., 2021c) and (Huang et al., 2015)).
maximise the average bubble film thickness up to 47 μm, which was nanoparticles with an ideal contact angle in a range of 40–80◦ is
213.7% higher than the reference group without XG. The use of 0.5% XG necessitated (Yekeen et al., 2018; Li et al., 2016). The wettability of
reduced the bleeding rate by 11.1% and smaller pore size and thus nanoparticles can be modified and shows a strong dependency on the
improved the 28-day compressive strength by 48.5%. surfactant types and concentration. In general, the individual adsorption
According to a study by Huang et al. (2015), collapse and air-voids of surfactant molecules on nanoparticles at low surfactant concentration
escape are successfully averted by the combined use of a thickening is mainly governed by electrostatic interactions between nanoparticle
agent and foam-stabilising emulsion. In their study, a thickening agent surface and oppositely-charged heads of ionic surfactants (Arab et al.,
comprised of a mixture of XG and cellulose ether was used to maintain 2018). Full monolayer adsorption of foaming surfactant molecules re
the consistency of the foam mix and increase the shell thickness of air sults in an increment of hydrophobicity of nanoparticle surface, thereby
bubbles. Meanwhile, foam stabilising emulsion synthesised by stearic promoting the mitigation of surfactant-adsorbed nanoparticles from the
acid, NaOH, KOH and ammonia aided in forming an electrical double liquid phase towards the gas-liquid interface. Excessive surfactant con
layer at the gas-liquid interface and accelerating the setting of foam centration beyond the critical threshold, nonetheless, triggers the for
cement paste. At the same amount of thickening agent, increasing the mation of the bilayer with the chain-chain association, rendering
content of foam stabilising emulsion exhibited high foam stability with bilayer-coated nanoparticles more hydrophilic (Arab et al., 2018).
finer pore size distribution (Fig. 8b). Zhang et al. (2021a) employed 8% Hence, a selection of appropriate concentration, types and compatibility
emulsified ethylene-vinyl acetate (EVA) copolymer as a polymer modi between foaming surfactants and particles strongly determine the de
fier, which produced polymeric film to solidify the gas-liquid interface gree of the presence and interaction of surfactant-adsorbed nano
and intertwine cement hydrates. The combined use of EVA copolymer, particles at the gas-liquid interface.
PCE superplasticizer, and waste-derived C–S–H was reported to trigger a Hou et al. (2019) reported that at the high concentration, hydro
synergetic effect for high foam stability and enhancement of mechanical phobic graphene nanomaterials (θ = 148.3◦ ) exhibited less efficiency
properties of foam concrete. and compatibility with a hydrolytic protein-based foaming agent than
that of hydrophilic nano-SiO2 (θ = 22.5◦ ). The presence of nano-SiO2 in
3.3.3. Role of ultra-fine particles or nanomaterials the hydrolytic protein-based foaming liquid phase increases the fluid
Several reports demonstrated that the nanomaterial adsorption in viscosity, providing finer pore and further uniform distribution than that
the form of monolayer, multi-layers or networks of clusters at lamellae of hydrophobic graphene. Further increasing the nanoparticle concen
and plateau border had been found to prompt a thick solid film around tration beyond this threshold was prone to decrease the strength of foam
the bubbles and high viscosity to counteract against kinetics of foam concrete. Abd Elrahman et al. (Abd Elrahman et al., 2021) concluded
destabilisation (Hunter et al., 2008). The adhesion/detachment energy that introducing nano-SiO2 by 5–10% improved the wall thickness of
between adsorbed nanoparticles and the gas-liquid interface is usually foam bubble, isotropic distribution of pores, and pore refinement,
on the scale of 103 kT, which is significantly greater than that of a sur thereby increasing the compressive strength of foam concrete by
factant molecule (approx. 1 kT) (Yekeen et al., 2018). The relationship 20–25% at 28 days. She et al. (2018a) reported that the oxygen anions
between the minimum particle detachment energy from the gas-liquid on the surface of nano-SiO2 could firmly bind with cations in the hy
interface of the foams (Ed), the particle radius (R), the interface/sur drophilic groups of amphoteric surfactant via electrostatic Coulomb
face tension (γ αβ) and the particle contact angle at the interface (θ), is forces, facilitating the adsorption of colloidal particles on the surface of
given by equation (1). bubbles. The redundant nano-SiO2 in bulk solution led to a substantial
increment of viscosity which strengthened bubble film and the blocking
Ed ≈ πR2 γ αβ (1 − |cosθ|)2 (1) of Plateau borders against gravity-driven drainage. These aforemen
Most surface-active nanoparticles, which are firmly held in the tioned effects triggered the modification pore structure of foam concrete
interface, possess a contact angle close to 90◦ , corresponding to the with narrower diameter distributions and lower mean bubble sizes
highest magnitude of adhesion/detachment energy. Nanoparticles with (Fig. 9). In particular, the pore diameter distribution declined from
extreme hydrophilicity or hydrophobicity (i.e. θ ≤ 30◦ and θ ≥ 150◦ ) 32–277 μm to 11–255 μm, 3–204 μm and 2–113 μm, corresponding to an
tend to get into either a suspended state in the liquid solution or a super- increasing addition of nano-SiO2 from 1% to 3% and 5%. A reduction in
liquid-repellent state (Cui et al., 2010). Consequently, it both exhibits mean pore size from 90 μm to 50, 40 and 30 μm was also recorded when
relatively low adhesion energy at the interface to form a stable foam. To introducing 1–5% nano-SiO2 to the mixture. However, it should be
achieve optimum foam stability, the incorporation of semi-hydrophobic noted that excessive flocculation of nanoparticles in the vicinity of the
10
Fig. 9. Foam microstructure at t = 0: (a) 0% nano-SiO2, (b) 1% nano-SiO2, (c) 3% nano-SiO2, (d) 5% nano-SiO2. The red scale bar of 200 μm is presented (Adapted
from (She et al., 2018a)).
gas-liquid interface may cause the foam to burst, thereby increasing the Furthermore, with the seeding effect, ultrafine or nanoparticles aid
mean pore size and number of interconnected pores. in promoting further hydration by providing nucleation points for pre
Another aspect that should be considered to achieve high foam sta cipitation and growth of hydrate phases (Wang et al., 2020a). The
bility is the compatibility of nanoparticle-surfactant pairs. It is reported pozzolanic reaction between nano-SiO2 with calcium hydroxide – Ca
that cationic and zwitterionic surfactants showed good compatibility (OH)2, according to the chemical equation (2), can form a layer of hy
with hydrophilic nanoparticles through electrostatic interactions. In dration products inside the pore, which enhance the rigidity of the cell
contrast, anionic surfactants are strongly paired with (semi)hydropho wall and mechanical properties of foam concrete (Krämer et al., 2015a).
bic nanoparticles due to hydrophobic interactions (Briceño-Ahumada For the use of nano-Al2O3, the early formation of ettringite can be
et al., 2021). Meanwhile, functional groups of nonionic surfactants (e.g. derived from a dissolution of nano-Al2O3 in a high pH alkaline solution,
ethoxylated or alkyl chains) may establish hydrogen bonding or hy according to equations (3) and (4) (Zhou et al., 2019). This aids in
drophobic interaction with the surface of nanoparticles (Briceño-Ahu promoting the densification of pore walls and considerable strength
mada et al., 2021). There are still limited reports analysing their development of foam concrete, especially at an early age.
compatibility in foam concrete; thus, further investigation is required to
2SiO2 (s) + 3Ca(OH)2 (s) + mH2 O(l) →3CaO⋅2SiO2 ⋅(m + 3)H2 O(s) (2)
gain more understanding.
Fig. 10. Morphology and EDS spectrum of the hydrated products of foam concrete (a) without nano-SiO2, and (b) with 5% nano-SiO2 (Adapted from (She
et al., 2018a)).
11
−
Al2 O3 ⋅ xH2 O(s) + 2OH(aq) →2Al(OH)−4 (aq) + (x − 3)H2 O(l) (3) and thus strengthened the liquid film (Fig. 11). Foam stability resulted in
evenly distributed pores with narrow size, which accounted for the
higher mechanical properties. Meanwhile, higher pH concentration led
2Al(OH)−4 (aq) + 6Ca2+ 2− −
(aq) + 3SO4 (aq) + 4OH(aq)
to the homogeneity of pore size (see Fig. 11). The salinity environment
+ 26H2 O(l) →3CaO⋅Al2 O3 ⋅3CaSO4 ⋅32H2 O(s) (4) with the presence of inorganic electrolytes is also an essential factor to
consider since it can influence the adsorption of surfactant molecules
The investigation of Kramer et al. (Krämer et al., 2015b) exhibited
and induce the failure of foam bubble (Li et al., 2011). For example, the
the effect of nano-SiO2 on the formation of hydrate phases surrounding
presence of Ca2+ and K+ electrolytes from pore solution or salts (e.g.
air pores. The van der Waals interaction between nanoparticles and
CaCl2, Ca(OH)2, KOH) trigger calcium or potassium chelation to depress
surfactant molecules draws them inside the pore walls. Besides the hy
the anionic surfactant, while it shows a negligible effect on cationic and
drate phases of clinker, Ca2+ ions could be transported through the
nonionic foaming agent (Qiao et al., 2020; Souza et al., 2017).
liquid solution and thus trigger the pozzolanic reaction with nano-SiO2
Decreasing the packing density of surfactant molecules at the gas-liquid
to form the secondary C–S–H phases. The thin layer of Ca(OH)2 crystals
interface led to a destabilisation of foam bubbles (Qiao et al., 2020). On
was mainly found on the inner pore due to the ample space for its
the other hand, Siva et al. (2015) revealed that the inclusion of sodium
growth, whereas the low-density C–S–H phases covered the exterior
salts (e.g. NaCl, Na2CO3, NaOH) increased the viscosity of foam liquid
surface of the pore. She et al. (2018a) also observed that a large amount
and maintained high foam stability with anionic foaming surfactant.
of plate-like portlandite crystals was primarily found in the pore walls,
Water repellent additives such as potassium calcium stearate (CS), pol
especially for foam concrete without nano-SiO2. Meanwhile, the pres
ysiloxane (PS), trimethylsilanolate (PT), re-dispersible latex powder
ence of nano-SiO2 at the gas-liquid interface could involve the pozzo
(RDL), sodium oleate (SO), siloxane-based polymer (SP), and zinc
lanic reaction to generate secondary C–S–H phases, which led to a
stearate (ZS) tended to reduce water absorption and connected pores,
greater uptake of Ca(OH)2 and a decline in Ca/Si ratio from 2.27 to 1.92
thus improve the strength properties of foam concrete when it was
(Fig. 10). As a result, smaller pore size together with densification of the
introduced in the mix by approx. 1–2 wt% (Ma and Chen, 2016; Liu
cell wall pore corresponding to the use of nano-SiO2 contributed to an
et al., 2019b; Záleská et al., 2019).
improvement of 28-day compressive strength by 1.8 times of the control
In addition, the foamability is affected by the impurities (e.g.
foam concrete. Gong et al. (2020) also pointed out that the foam con
macromolecular materials), which can strongly interact with ionic sur
crete incorporating 4% of nano-SiO2 attained flaky CH crystals, denser
factants and polar organic additives in the concrete mixture (Du and
ettringite, high C–S–H gels and a negligible quantity of unhydrated
Folliard, 2005). For example, organics in algae-contaminated water can
particles. This resulted in a significant improvement in the 28-day
stabilise air bubbles, whereas high concentrations of Ca2+ and Mg2+ in
compressive strength of foam concrete (ds = 1600 kg/m3), from 26.9
hard water decrease the air content (Du and Folliard, 2005). Notably,
MPa to 35.8 MPa (approx. 33%), and 28-day flexural strength by 24%.
using magnetised water instead of regular tap water in the foaming
The early-age 72-h autogenous shrinkage was mitigated by 50.2%;
process can strengthen a hydrogen bond between magnetic water and
however, an increase in drying shrinkage by 170.7% after 56 days was
foaming surfactant molecules due to the intermolecular forces (Ghor
recorded. A similar effect of nano-SiO2 on strength improvement of
bani et al., 2019a). In addition, the magnetic force lessens the size and
cellular concrete up to 22% was also reported by Yu et al. (2015).
number of water molecule clusters, thereby facilitating thicker lamellae
Xiong et al. (2021) reported that the carboxyl group of SOCl2-treated
plateau borders (Ghorbani et al., 2019b). Increasing the number of
synthetic surfactant strongly bonded with the hydroxyl group on the
passing through a permanent magnetic field render the structure of
nano-Al2O3 to establish an uniform distribution of nanoparticles onto
water molecules further stable and ordered, which increases the foam
the gas-liquid interface. In the hardening phase, pozzolanic reaction
stability and homogeneous microstructure (Ghorbani et al., 2019a,
resulted in significant uptake of Ca(OH)2, corresponding to the use of
2019b; Karpenko et al., 2020). This led to an improvement in
nano-modified synthetic surfactant. Protein-based foaming surfactant,
compressive and splitting tensile strength up to 40% and 50% at 28 days
however, showed less compatibility with nano-Al2O3 due to a high
and a reduction in water absorption of 15% (Ghorbani et al., 2019b).
quantity of Ca(OH)2 and CaCO3, which was found in the vicinity of the
However, it should be noticed that magnetic water is prone to be suit
pore wall. Notably, nano-modified foam concrete still exhibited an
able for synthetic rather than natural foaming agents.
enhancement in mechanical strength and shrinkage. Furthermore, the
integration of three-phase foams with oxidised carbon nanotubes (CNTs)
3.3.5. Effect of gas types
and titanate nanotubes (TiNTs) could further facilitate the increment of
Notably, foamability and foam stability also depend on the types of
crystallinity and strength improvement of foam concrete. TiNTs have a
gas. Air, hydrogen, oxygen, and carbon dioxide are the popular gases
high Ca2+ affinity and provide a better seeding effect for the precipita
used for foam generation, corresponding to the foaming methods and
tion of C–S–H phases than CNTs (Krämer et al., 2016a, 2016b).
purpose of use. Void features containing O2 and H2 gases in chemically-
Multi-walled CNTs can refine the pore size and enhance pore structure
foamed concrete (dp = 0.5–3 mm, spherical shape) often possess a
and strength (Luo et al., 2017). Also, the bridging effect of nanotubes in
coarser size but more spherical shape than those in mechanical foaming
the nano scale was supposed to contribute to suppressing nano/
(dp = 0.1–1 mm, less spherical shape) (Zhang et al., 2014; Nambiar and
microcracks for substantially improving the ultimate strain, toughness,
Ramamurthy, 2007b; Akthar and Evans, 2010). Also, these foaming
peak strength and post-peak compressive resistance of foam concrete
gases possess different thermal conductivity, for example, air (0.0242
after deformation (Luo et al., 2017; Zhang et al., 2020d; Baloch et al.,
W/mK), H2 (0.1760 W/mK), O2 (0.0240 W/mK) and CO2 (0.0143
2018; Zhang and Liu, 2019; Kerienė et al., 2013; Wang et al., 2020b).
W/mK). Li et al. (2020a) stated that the thermal properties of foam
concrete are proportional to the thermal characteristics of foaming
3.3.4. Effect of water/solution salinity
gases. Using carbon dioxide as foaming gas, Foam concrete achieved the
The change in the pH and salinity of foam liquid solution is critical
lowest thermal conductivity compared to air, O2, or H2 gas. The low
for foam stability (Tarasenko, 2021). Depending on surfactant types and
solubility of gas (e.g. fluorinated gases) is also an intriguing aspect that
charges, the pH of solutions can influence the geometric structure of
can impede the gas transfer within the matrix during the coarsening
surfactant molecules and foam stability (Sahu et al., 2018). Yuanliang
process. Despite an interesting perspective, limited studies investigate
et al. (2021) modified the alkaline concentration in a liquid solution by
the effect of gas types on pore structure or rheology of foam concrete in
employing Ca(OH)2 as a foam stabiliser. The pH of the solution was
the fresh state. Further research should be conducted to provide an
adjusted from 9 to 12.5. The authors pointed out the effect of Ca(OH)2
understanding of its effects.
concentration at a pH of 9, which increased the viscosity of the foam mix
12
Fig. 11. Effect of Ca(OH)2 concentration on the rheological behaviour and pore structure of the cementitious matrix. (Adapted from (Yuanliang et al., 2021)).
3.4. Internal reinforcements hydrophilicity and capillary pores than the non-treated fibres. As a
result, treated kenaf fibres aided in decreasing the shrinkage rate of
The incorporation of fibres as internal reinforcements in foam con foam concrete by approx. 24%, but experienced brittle fracture after the
crete contributes to the enhanced capacity of the composite for tensile initial cracking. Castillo-Lara et al. (2020) pointed out that adding
stress uptake. Manifold types of fibres from man-made to natural origin treated henequen fibres considerably increased the compressive and
has been utilised in foam concrete, including kenaf (Mahzabin et al., tensile strength, toughness and ductility of foam concrete without
2018; Awang and Ahmad, 2014), coir (Zhang et al., 2020e, 2020f), sisal modifying the pore structures.
(Krishna et al., 2018), henequen (Castillo-Lara et al., 2020), hemp It should be noticed that pore formation is prone to be affected by the
(Gencel et al., 2021b), sugarcane bagasse (Madhwani et al., 2021), size of fibre (Tran et al., 2022a, 2022c). Microfibre networks over
basalt (Sinica et al., 2014; Pehlivanlı et al., 2016), kaoline (Laukaitis lapping each other act as internal barriers to limit the free movement of
et al., 2009), polypropylene (PP) (Mugahed et al., 2020; Jones and the air bubbles within the matrix and subdivide macropores into finer
McCarthy, 2005b; Liu et al., 2022a; Mamun et al., 2014; Yang et al., mesopores (Tran et al., 2021b, 2022b). This refinement effect, together
2022), polyvinyl alcohol (PVA) (Raj et al., 2020; Masi et al., 2015), with the bridging action of microfibres, contributes to the enhancement
polyolefin (Rasheed and Prakash, 2015; Rasheed et al., 2018), steel of the strength properties of foam concrete. In particular, Bing et al.
(Wang et al., 2019b; Rafal Ahmed and Abed, 2021), carbon (Laukaitis (2012) developed a high-strength fibre-reinforced foam concrete of
et al., 2012; Namsone et al., 2017), and glass fibres (Dawood and 800–1500 kg/m3 with a corresponding strength of approx. 10–50 MPa
Hamad, 2015; Pehlivanli et al., 2016; Calis et al., 2021). Reportedly, the (Fig. 12). In all investigated matrix combinations, the addition of 0.8%
volumetric fraction of the fibre reinforcement should be kept below PP microfibres (l = 15 mm, d = 100 μm) showed an increment in 90-day
0.4% of the total volume of the designed foam mixture to avert the fibre compressive strength by 16–45%, 90-day splitting tensile strength by
agglomeration and settlement of fibre bundles (Amran et al., 2015; Raj 31.7–50%, and shrinkage reduction by approx. 50% at 90 days for
et al., 2019). Also, characteristics of fibres – such as size, shape, geom different concrete densities. Likewise, Irawan et al. (2019) reported a
etry, wettability and modulus of elasticity, play a vital role in influ significant improvement in the strength properties of foam concrete,
encing fibre-matrix interfacial properties, pore distribution and overall corresponding to the inclusion of 0.4% of PP microfibres (l = 12 mm, d
performance of foam concrete. Hooked-end geometry of steel fibres = 18 μm). The compressive strength of PP microfibre reinforced foam
demonstrates a superior anchoring effect for strength improvement of concrete was approx. threefold that of the control with an increase from
foam concrete than the corrugated (Rafal Ahmed and Abed, 2021). 2.85 MPa to 8.38 MPa, while an increment of 5.5 times from 0.5 MPa to
However, some indicated the incompatibility between heavyweight 3.23 was also recorded for flexural strength at the same age of 28 days.
steel fibres and lightweight foam base that could cause gravitational In addition, Amran et al. (Mamun et al., 2014) revealed that PP
segregation between foam floating upward and fibres sinking at the microfibres’ effect on the strength improvement of foam concrete was
bottom (Amran et al., 2015; Wu et al., 2020). Accordingly, polymeric further pronounced for those with lower density. Castillo-Lara et al.
and natural fibres with lower density have been widely opted to rein (2020) observed a negligible change in pore structure surrounding a
force foam concrete. single PP microfibre; however, large air voids were found in the vicinity
Due to the porous structure of cellulose and lignin, plant-based fibres of fibre bundles. This indicated the importance of good dispersion of
possess highly-hydrophilic natures. Moisture absorption of these natural microfibres throughout the matrix without disturbing the compactness
fibres reduces flowability when introduced into foam mixtures, thereby and foam stability. Several reports also showed a considerable strength
impeding the coalescence of air bubbles. In particular, it was observed enhancement, reduction in shrinkage and thermal conductivity when
that a fine and closed pore structure with uniform distribution of pore introduced PP, PVA, carbon and glass microfibres in foam concrete
size corresponded to the inclusion of hydrophilic coir and chrysotile (Castillo-Lara et al., 2020; Raj et al., 2020; Batool and Bindiganavile,
asbestos fibres in foam concrete (Raj et al., 2020). Reportedly, this pore 2020b; Chen and Wang, 2018).
refinement effect correlated to a significant improvement in strength In addition, Dawood and Hamad (2015) pointed out that the high
properties and thermal conductivity of foam concrete. To amplify the stiffness of glass fibres displayed outstanding performance in strength
effect of natural fibres in foam concrete, fibre treatment has been ening foam concrete than the low stiffness of polypropylene fibres. The
employed to neutralise its hydrophilicity and increase the surface addition of 0.6% glass microfibre (l = 12 mm, d = 14 μm) substantially
roughness for better anchoring. According to the study of Mahzabin increased 90-day compressive strength by 51%, flexural strength by
et al. (2018), alkaline-treated kenaf fibres exhibited lesser 21% and elastic modulus by 48.8% at 28 days. Although the use of
13
Fig. 12. Effect of PP fibres on compressive strength of foam concrete at 28 days (Adapted from (Bing et al., 2012)).
polypropylene macrofibres (l = 45 mm, d = 1 mm) did not contribute to MK-based geopolymer foam can be cured at the ambient condition,
strength improvement, its presence was more efficient in strengthening while FA precursor requires heat curing (>40 ◦ C) to proceed with the
foam concrete’s toughness and post-crack behaviour. geopolymerisation (Dhasindrakrishna et al., 2021a). For foam mixes
To compensate the shortcomings of each fibre type and size, fibre using chemical foaming, the foaming process is sensitive to the tem
hybridisation is considered a pragmatic approach. Rasheed and Prakash perature as heat curing expedites the setting of binder and gas release
(2015) concluded that the synergetic effect of polyolefin macrofibres (Huang et al., 2015). The appropriate curing heat to balance between
and fibrillated fibres played a vital role in bridging the micro and these two reaction rates is crucial to not only facilitate the foaming
macrocracks. This led to a considerable enhancement of compressive process but also hinder the coalescence and collapse of bubbles (Medri
and flexural strength by 117% and 69.6%. The addition of a small and Ruffini, 2012). To optimise foaming process and pore structure of
quantity of microfibres in hybrid fibres remarkably enhanced the geopolymer foam, periodical cycles of thermal curing with varying
toughness, fracture behaviour and ductility of foam concrete compared temperatures or microwave irradiation are potentially considered in lieu
to the inclusion of only macrofibres (Rasheed et al., 2018; Rasheed and of fixed curing temperature (Henon et al., 2012; Nadeem et al., 2020).
Prakash, 2018). Other combinations, such as PVA and coir fibres, steel Regarding cement foam, water curing provides the ideal condition
and PP fibres, also provided a synergetic effect to improve the thermal for complete cement hydration instead of other curing methods. Gökçe
resistance and strength of foam concrete (Raj et al., 2020; Rafal Ahmed et al. (2019) found that there was a comparable compressive strength of
and Abed, 2021). Notably, Falliano et al., 2019a, 2019b employed foam concrete subjected to 7-day water curing (t = 20 ◦ C) and autoclave
glass-fibre-reinforced-polymer (GFRP) mesh (4 × 4 mm2 spacing) in the curing (t = 200 ◦ C, p = 1.4 MPa) in 3 h. The strength enhancement effect
tensile zone, together with 2–5% of short polymer macrofibres (l = 20 of SF was more pronounced in autoclave-cured specimens due to the
mm, d = 0.54 mm). The result findings revealed a significant additional hydration of unhydrated cement grains. Although steam
improvement in flexural capacity by up to 10–15 times the reference curing accelerates the strength gain within 24 h, water-cured foam
beam at the same density of 400 kg/m3. The magnitude of flexural concrete at later ages tends to achieve higher compressive strength than
strength improvement tended to be lessened for higher densities of those with autoclave curing (Pan et al., 2014). In some cases,
600–800 kg/m3. Shrinkage of foam concrete was mitigated by 35–45% cellophane-sheet-sealed curing was found to be further effective for
at 28 days, corresponding to the presence of 2–5% short fibres. Despite strength improvement of foam concrete instead of air and water curing
the use of high fibre content, it exhibited a negligible influence on the when its dry density was lower than 400 kg/m3 (Falliano et al., 2018,
compressive strength of foam concrete. 2019a). Carbonation curing is also an effective approach to sequestrate
CO2 and enhances compressive strength (Guo et al., 2019). The crys
talline carbonation products such as calcite and aragonite interlace the
3.5. Alternation of curing/treatment regime pore and dense microstructure, resulting in higher properties.
Foam stabilisation can be further improved through several pre-
Technically, high temperature can not only shorten the induction treatment techniques such as microwave and ultrasonic treatment.
periods of foaming (Jin et al., 2021), but also promote the dissolution of Especially for a protein-based foaming agent, the absorption of micro
the anhydrous clinker to accelerate the setting and hardening process of wave energy affects the denaturation level of protein. It thus facilitates
foam paste (Wei et al., 2014; Lothenbach et al., 2007). Increasing the the exposure of hydrophobic chains for the adsorption of molecules
casting temperature up to 40 ◦ C leads to improving the fluidity of foam (Kuzielová et al., 2016). Treated protein-based foam concrete obtains
paste slurries with smaller pore size and homogeneous, isotropic, smaller pore size and higher compressive strength (Kuzielová et al.,
spherical foam bubbles (Jin et al., 2021). The compressive strength of 2016). Microwave technology in the pre-curing process is considered as
foam concrete mounts up to the optimum value when the temperature a potential approach to improve the stability of pores and the overall
reaches 45 ◦ C (Huang et al., 2015). It should be noted that high curing performance (Cai et al., 2020; Tang et al., 2020).
temperature has a substantial influence on the strength development of She et al. (2020) developed a nanohybrid method in which nano-
foam mixtures incorporating high volume FA due to the heat-induced SiO2 particles were grafted with hydrophobic functional groups to
acceleration of its pozzolanic reaction (Liu and Zhang, 2021; Zhang promote the adsorption of these modified nanoparticles onto the
et al., 2018b; Kearsley and Wainwright, 2002a; Lee et al., 2020). For gas-liquid interfaces. The combined effect of modified nanoparticles on
alkali-activated foam concrete, a curing temperature range of 20–80 ◦ C the roughness and hydrophobicity of the gas-liquid interface provided
is commonly adopted, while the higher temperature is deemed not an an enhancement of mechanical characteristics and thermal insulation
energy-efficient approach (Dhasindrakrishna et al., 2021a). Slag and
14
for foam concrete. Another approach involves the application of ultra

fine fly-ash-based superhydrophobic composite coating via spraying Furthermore, aerogel has proved to be a promising approach to
technique (Song et al., 2020). The coated surface of foam concrete can producing an energy-efficient, high-performance foam composite
exhibit waterproofing behaviour and highly-efficient heat preservation (Lamy-Mendes et al., 2021; Adhikary et al., 2021; Abu-Jdayil et al.,
(Song et al., 2020). Song et al. (2021b) applied a lithium silicate 2019; Do et al., 2021). A novel sol-gel technique with vacuum impreg
impregnation method to treat hardened foam concrete after 7 and nation into SiO2 aerogel solution and an ethanol supercritical drying
28-day water curing. Lithium silicate could react with Ca(OH)2 to form process was employed by Liu et al. (2018) to reinforce foam concrete
new C–S–H phases for improving strength and hardness, as shown in (Fig. 13a). SiO2 aerogel attached and filled in the open pores of foam mix
equation (5). The foam concrete treated with lithium silicate solution for to provide the nano-porous network therein, giving distinct structural
6 h possessed a higher proportion of meso and micropores but fewer characteristics for enhancing thermal performance and energy-saving
macropores in comparison to the non-treated foam concrete. This capacity. Likewise, Yoon et al. (2020) applied the sol-gel method to
approach helps achieve a relatively high strength gain for foam con manufacture hydrophobic nano-aerogel foam concrete. The compounds
crete, up to 59.5%. used in nano-aerogel solution include the single form of methyl
trimethoxysilane (MTMS) and the blended form of MTMS and tetraethyl
Li2O⋅nSiO2 + mH2O + nCa(OH)2 → nCaO⋅SiO2⋅(m + n − 1)H2O + 2LiOH (5)
Fig. 13. (a) Schematic diagram of structural evolution of an aerogel-foam concrete system, and (b) contact angle testing of modified foam concrete (Adapted from
(Liu et al., 2018; Yoon et al., 2020)).
15
orthosilicate (TEOS). The uniform dispersion of nano-aerogel and the high-stress concentration and crack initiation mainly occurs in the
flocculation of hydrophobic nanoparticles on the interfacial substrate of narrow interstitial border, dense particle packing in these regions
air voids resulted in a lower thermal conductivity of 13–18% and higher against gravity-driven settlement contributes to strength development
hydrophobicity of surface (Fig. 13b). TEOS could be hydrolysed into for higher load-bearing capacity (Chung et al., 2017).
SiO2–H2O gels to fill the capillary pores (Luo et al., 2017). This As summarised in Table 1, different strategies contribute to foam
addressed a rapid moisture absorption observed for the conventional stability and enhance the performance of foam concrete. The selection of
foam concrete caused by a high quantity of interconnected pores and foaming surfactants, polymer-based admixtures, and nanomaterials are
enhanced its anti-permeability (Luo et al., 2017; Yoon et al., 2020). Shi crucial strategies directly influencing the interfacial stabilisation of
et al. (Hilal et al., 2015c) also employed the conventional sol-gel method bubbles. In addition, the optimisation of binders with additives facili
to treat foam concrete with isobutyl triethoxysilane and graphene oxi tates proper yield stress to constrain bubble sizes and maximises the
de/silane (GO/IBTS) composite emulsion. Strong bonding between strength development in the later ages. The curing regime maximises
GO/IBTS and cementitious matrix forms a uniform waterproof film on strength development; meanwhile, surface treatment improves the
the surface, reducing water absorption by 9.3% and possessing excellent durability of foam concrete. The addition of either fibres, accelerators or
superhydrophobic properties. Likewise, high-quality foam concrete aggregate substitutes provides reinforcing and pore refining effects,
could be achieved by applying surface treatment with silica and which increases the load-bearing capacity of foam concrete. On the
iron-bearing sol solution (Svatovskaya et al., 2016) or GO/silane (Geng other hand, using different salts (e.g. CaCl2, KOH) and gases is deemed
et al., 2020; Gao et al., 2021). less effective than other strategies.
High-performance foam concrete can achieve equivalent or even
4. Perspectives and discussion higher strength in comparison to autoclaved cellular concrete (Fig. 14a).
Density minimisation is desirable to achieve a lower self-weight, water
Achieving the desired strength of at least 10–40 MPa for a density of absorption and thermal conductivity (Gökçe et al., 2019); however, this
less than 1600 kg/m3 necessitates the highly-stable foam to control the tendency leads to an exponential compensation of strength properties
pore topology, together with a high-strength border surrounding the (Kearsley and Wainwright, 2002b). The optimum density should be
pores. The former can be governed by the yield stress of interstitial paste reached at a rational range to balance the porosity-related properties.
and the modified bubble-matrix interface. Proper yield stress of matrix Gokce et al. (Gökçe et al., 2019) plotted the curves from the data set of
slurry, which satisfies foam stability criterion (Feneuil et al., 2019a, water absorption and compressive strength to determine that the opti
2019b), provides sufficient confinement force (see Fig. 4a) to isolate mum density range for structural foam concrete should be beyond 1320
individual bubbles in the form of closed pore shells and hinder its kg/m3 (Fig. 14b). Accordingly, foam concrete with a density of 1424
buoyancy-driven motion. The rheology of mixtures can be governed by kg/m3, close to the optimum value, obtained compressive strength of
optimising the mix components (e.g. particle size distribution, chemical 26.8 MPa (Gökçe et al., 2019), which is suited for structural applications
composition, shape) that reflects the pore refinement (Xie et al., 2018) in accordance with ACI 213R-03 (ACI Committee 213R-03, 2003).
and the strength of foam concrete (Hilal et al., 2015c). Meanwhile, the Amran et al. (Mugahed et al., 2020) developed high-strength foam
strong interstitial borders are liable to attain by employing the models of concrete incorporating FA, SF additives and PP fibres, which reached the
dense particle packing in the mixture design (Krämer et al., 2017). Since 28-day strength of up to 30 MPa, 50 MPa and 70 MPa, corresponding to
Table 1
Evaluation of strategies for foam stabilisation.
Strategies Constituent materials Effects Degree of Remarks
effectiveness
Modification of the gas-liquid Nanomaterials Bubble stabilising High Excellent performance but high cost
interfacial structure Nucleation seeding
Pozzolanic
strengthening
Pore refining
Polymer chains Water reducing High Benefits of an appropriate dosage of SP
Rheology modifying A thickening agent is highly recommended
Pore refining
Surfactants Bubble stabilising High Types of charges and compatibility with binders
Foaming
Alkali/acid salts pH controlling Low Limited amount to facilitate foaming process
Bubble stabilising
Gases Foaming Low Depending on foaming techniques
Pore refining
Optimal selection of binder Binders, water & Rheology modifying High Composite binders are recommended to achieve desirable yield
combination solutions Strength promoting stress
Mineral additives Rheology modifying High Dosage is dependent on the characteristics of additives
Pozzolanic Reactive pozzolans are recommended
strengthening
Pore refining
Accelerators Rheology modifying Medium Benefits with appropriate dosage and types
Quick setting time
Sand replacement by fillers Aggregates Rheology modifying Medium High fineness of fillers is recommended
Filling
Pore refining
Internal reinforcement Fibres Rheology modifying Medium Microfibres rather than macrofibres show better effects in pore
Bridging refinement
Pore refining
Curing and treatment methods – Moisture containing High Autoclave curing promotes strength development but does not
Strength promoting imply energy efficiency
Pore refining Surface treatment improves durability
16
Fig. 14. (a) Strength development and (b) Optimum density range for high-performance foam concrete (Adapted from (Gökçe et al., 2019), (Krämer et al., 2015b)).
Fig. 15. Schematic diagram of synergetic action mechanism of hardening accelerator A-3, ethylene-vinyl acetate emulsion (EVA), C–S–H seeds and PP fibres in FA-SF
blended foam concrete (Adapted from (Zhang et al., 2021a)).
the densities of 1300, 1600 and 1900 kg/m3. Technically, this light structures (Saheed et al., 2021). Apparently, mass production of iden
weight material has been proven to meet structural concrete’s basic tical building parts comprising lightweight foam concrete for the same
requirements (Mugahed et al., 2020). size units results in lower transportation, operation and labour cost.
It should be noticed that the combination of existing strategies even Those factors contribute to the fast-paced development and economic
displays high effectiveness and superior performance. Zhang et al. benefits for the prefabricated construction and building industry.
(2021a) pointed out that the synergetic action of multi-additives In practice, applications of cellular foam concrete vary based on its
contributed to stronger foam concrete with a considerable enhance densities and properties (Table 2). Due to insufficient strength, ultra-
ment of hardened properties than the use of individuals. In particular, lightweight foam mainly serves as thermal and acoustic insulators (Shi
the EVA emulsion solidified foam bubble into small size and intertwined et al., 2021). The most extensive use for infra-lightweight foam (<800
with cement hydrates; meanwhile, C–S–H seeds and accelerators pro kg/m3) is the production of sacrificial cladding of tunnel lining struc
moted hydration and the pozzolanic reaction of FA-SF-blended cement tures (Zhao et al., 2015b), road/bridge embankment (Huang et al.,
grains for enhancing foam stability and strength (Fig. 15). The presence 2017), pavement subbase (Ni et al., 2020), raft foundation (Mohd Sari
of PP fibres in the foam matrix also further improved the energy ab and Mohammed Sani, 2017), and AAC blocks (Wang et al., 2019c).
sorption capacity against stress-induced cracking. Similar observations Meanwhile, further (non)semi-structural building and construction ap
with a combined strategy have been reported by other researchers (Bing plications such as parapets (Mohd Sari and Mohammed Sani, 2017),
et al., 2012; Hilal et al., 2015b; Jones and McCarthy, 2005b). foam concrete bricks/blocks (Bhosale et al., 2020), shear walls (Li et al.,
2020b), high-speed railway roadbed (She et al., 2018c; Cai et al., 2021),
5. Trending applications coal mine roadway filling materials (Tan et al., 2017) are suitable for
foam concrete with higher densities (800–1200 kg/m3). For densities
Enhanced structural and energy efficiency in respect of strength-to- beyond 1350 kg/m3, foam concrete can be applied for sandwich panels
weight ratios and refined pore structure renders innovative foam con and load-bearing wall/slab/floors in precast or cast-in-place forms
crete versatile for various construction and building applications (Mugahed Amran et al., 2016, 2019). Several reports indicated that the
(Fig. 16). Due to inherent porosity, it ideally suits lightweight precast fire-proof wall panels made of foam concrete could achieve a compres
structures and prefabricated housing such as modular segmental fa sive strength of 5–25 MPa and thermal conductivity of 0.016 W/mK,
çades, sandwich panels, load-bearing walls and thin composite slabs. corresponding to densities of 800–1500 kg/m3 (Ching, 2012). When the
Cellular foam concrete is often 20–40% lighter than conventional con densities range from 1500 to 1850 kg/m3, the proper mix design of foam
crete with comparable strength. In lieu of using traditional materials, concrete can reach a compressive strength of up to 58 MPa (Amran et al.,
fabricated building elements (e.g. slabs, columns, beams) made of 2015).
structural foam concrete can minimise the dead loads, which account for In recent, emerging 3D printing technology facilitates a higher de
approx. 40–60% of the total vertical loads imposed by the building gree of customisation and architectural details with geometric
17
Fig. 16. Construction and building applications of foam/cellular concrete (Adapted from (Wu et al., 2020; Huang et al., 2017; Bhosale et al., 2020; Fu et al., 2020;
Light Concrete, 2003; Cho et al., 2022; Markin et al., 2021; Alghamdi and Neithalath, 2019)).
Table 2
Applications of foam concrete with respect to the densities and properties (Narayanan and Ramamurthy, 2000; Bing et al., 2012; Dhasindrakrishna et al., 2021a;
Mugahed et al., 2020; Mohd Sari and Mohammed Sani, 2017; Light Concrete, 2003).
Density (kg/ Compressive strength Elastic modulus Thermal conductivity (W/ Shrinkage Applications
m3) (MPa) (GPa) mK) (%)
100–300 <1 <1 <0.1 >0.3 ⁃ Thermal/acoustic insulators

300–800 1–12 1–4 0.1–0.2 0.2–0.3 ⁃ Soil stabilisation, raft foundation
⁃ Tunnel backfill, filling of sewerage pipes and wells.
⁃ Cladding, trench reinstatement and road/bridge
embankment
800–1200 2–27 2–6 0.2–0.4 1.1–0.2 ⁃ Balcony railing, partition, parapets, and bricks/blocks
⁃ High-speed railway roadbeds, basement and subways
⁃ 3D printable foam concrete
1200–1850 4–60 4–18 0.4–0.8 <0.1 ⁃ Prefabricated sandwich panels and shear walls
⁃ Load-bearing slabs
⁃ Precast columns and beams
⁃ High-stiffness 3DP foam
complexity and no extra cost from mould prefabrication and tooling at a density beyond 600 kg/m3, exhibit no settlement and lateral
(Bedarf et al., 2021; Ngo et al., 2018; Liu et al., 2021e; Eugenin et al., expansion compared to traditional cast foam concrete (Falliano et al.,
2021). Due to high consistency, the extruded foam concrete, especially 2020b). The 3D printing process facilitates the adjustment and
18
customisation of the printing parts with varying densities, correspond foam concrete technology over the traditional casting method, foam
ing to specific areas subjected to either intensive stresses or high thermal stability under built-up pressure within the nozzle or extruder is not yet
gradients (Bedarf et al., 2021). Hence, 3DP foam concrete can be comprehended. The density discrepancy across the extruded filaments
potentially opted to produce load-bearing walls in the multi-story house causes uneven stress and local deformation, posing the main challenge
at a low cost (Markin et al., 2019). The latest research has proved the for digital foam concrete applications.
potential for insulation improvement by using 3DP foam concrete Energy efficiency and economic feasibility – To sustain a low carbon
printed onto the existing façades and walls (Lublasser et al., 2018). Also, footprint, energy-related curing (e.g. autoclave, heat) and ordinary
3D printing technology extends the conventional design of modular cement must be avoided. This practice, however, can affect the hard
foam elements such as façades, wallboards, blocks, and sandwich panels ened properties of foam concrete. The high cost of nanomaterials,
to a broader range of architectural freeform envelopes (Bedarf et al., chemical admixture or alkaline solutions present a challenge to be
2021). For example, a buoyant façade element that was 2.1 times lighter widely adopted in practice.
was developed by 3D printing, proposing a practical alternative to
conventional concrete (Cho et al., 2021). Notably, Markin et al. (2019) 7. Concluding remarks
successfully developed free-form 3DP foam concrete with 28-day
compressive strength of 10.4 MPa, corresponding to a dry density of This paper presents a comprehensive review of the recent progress,
980 kg/m3. Alghamdi and Neithalath (2019) printed the foam geo state-of-the-art techniques and new advancements for foam stabilisation
polymer with a good buildability and apparent density lower than 1000 in a bid to maximise the performance of foam concrete. Sustainable
kg/m3. With outstanding features of 3DP foam concrete, it has applications and potential challenges have been highlighted for future
increasingly gained the attention of researchers so far (Liu et al., 2021d, research and development. The main conclusions can be drawn as
2022b; Cho et al., 2020, 2022; Alghamdi and Neithalath, 2019; Falliano follows:
et al., 2020b; Markin et al., 2019; Lublasser et al., 2018).
1. High-strength borders surrounding the pores and uniform and
6. Potential challenges refined pore characteristics are the key features of high-performance
foam concrete. To yield strong interstitial borders, the models of
Despite the benefits of cellular structure, several drawbacks still exist dense particle packing with nanomaterials or ultra-fine particles
that necessitate further research and technological innovation to render must be applied in the mixture design. Proper dosage of semi-
promising foam building materials prevalent in real-world construction hydrophobic nanoparticles facilitates their adsorption onto the
practices in lieu of a limited laboratory scale. Bringing these porous bubble interface and presence in the Plateau border to isolate the fine
building materials into the construction market faces many technical air bubbles and act as nucleation seeds for further hydration and
and industrial barriers due to the lack of tailored standards and concerns pozzolanic reaction in this region. Hardened pore structure strongly
about their long-term performance. Hence, several existing and poten reflects through the stability of fresh foam slurry, governed by yield
tial challenges should be addressed as follows: stress and setting of the base mix.
Mixing protocols – Foam stability and pore formation in the fresh 2. Types, chemical composition and fineness of binder determine the
state are deemed to be influenced by the mixing intensity and method (e. degree of stabilisation of foam slurry. The use of composite binders,
g. rotational speed, dry/wet mixing, time-dependent drainage) (Stel especially rapid hardening binders, exhibits synergetic effects to
makh et al., 2020; Shah et al., 2021b). The increase in mixing intensity accelerate high early strength development at borders of pores,
correlates to a significant increment of mechanical strength (Falliano thicken pore walls and refine pore structure. The use of limestone
et al., 2020b). In addition, no sign of segregation and less shrinkage is and other set-accelerators should be limited to avert their side effects
achieved by a dry mixing method instead of wet mixing (Safawi et al., on pore structure.
2021). Due to a lack of understanding of the mixing effect, the syn 3. The high fineness of fillers increases the yield stress through a
chrony in the mixing process and rest time are the challenges to con colloidal interaction of contacting particle networks, resulting in the
trolling the homogeneity of the foam mixtures. pore refinement with uniform pore distribution. This phenomenon is
Raw material optimisation – The combination of manifold binders, more pronounced by using pozzolans such as fly ash, palm oil fuel
nanomaterials, chemical admixtures and mineral additives exhibited ash and quarry dust. Angular and coarse fillers are deemed to cause
high complexity of hydration kinetic and depended on the chemical clustering of air bubbles that form interconnected pores with irreg
compositions of the raw materials. Quick settings of foam mix facilitate ular shapes.
small and uniform pore distribution; nonetheless, it is less likely to suit 4. Foaming surfactant concentration should be kept close to the critical
mass production where a longer setting time for processing is required. micelle concentration (CMC) threshold to stabilise bubbles and
In addition, a wide range of precursors and activators, which have maintain the appropriate viscosity of surfactant solution. In addition,
heterogeneity in chemistry and physics, induce difficulty in the quality the compatibility between foaming agents and cement grains is a
control of geopolymer foam (Zhang et al., 2014; Dhasindrakrishna et al., crucial factor for foam stabilisation, which is dependent on their
2021a). types of charges. The combination of foaming agents is a potential
Long-term durability – High alkalinity, efflorescence, corrosion and approach to achieve a synergetic effect on high foam stability.
shrinkage at high temperature of geopolymer foam (Zhang et al., 2021b; However, not all types of foaming agents are compatible with each
Wasim et al., 2021), as well as long-term durability of cementitious foam other (e.g. cationic vs. anionic surfactants). Chemical admixture such
against acid and frost attacks (Indu Siva Ranjani and Ramamurthy, as thickening agents should be employed to increase the lamellae
2012; He et al., 2021) should be taken under scrutiny corresponding to thickness around the bubbles; meanwhile, the dosage of super
different conditions. In addition, the thickness of the passive oxide layer plasticisers should be limited to approx. 0.1% w. t to maintain the
is liable to be lower than that in the non-foam base mixture due to the appropriate viscosity for high foam stability.
presence of voids in the vicinity of the steel bar (Bagheri and Rastegar, 5. The pore structure of foam concrete is affected by the wettability and
2019). This indicates a higher vulnerability of foam concrete against size of fibres as pore refinement is further pronounced by adding
corrosion under aggressive conditions. Despite having several fibres with micro size and hydrophilicity. Fibres with high stiffness/
surface-coating treatments to hinder the penetration of aggressive modulus display a remarkable reinforcing effect to strengthen the
agents, the complexity and high cost are still the barriers limiting the mechanical properties of foam concrete. In comparison to individual
structural applications of high-performance foam concrete. fibre, the addition of fibre hybridisation achieves further
Technological development – Despite the superior features of 3DP
19
enhancement in the load-bearing capacity of foam concrete due to CRediT authorship contribution statement
synergetic effects.
6. The change in the pH, impurities and salinity of foam liquid solution Nghia P. Tran: Conceptualization, Methodology, Software, Data
is critical for foam stability, disturbing the adsorption and interac curation, Formal analysis, Investigation, Visualization, Validation,
tion between ionic surfactant and polar organic additives. Salts such Writing – original draft. Tuan N. Nguyen: Supervision, Writing – review
as Ca(OH)2 can be employed as foam stabilisers to modify the alka & editing, Project administration. Tuan D. Ngo: Supervision, Writing –
line concentration in a liquid solution to be compatible with foaming review & editing, Project leader, Funding acquisition. Phung K. Le:
surfactant. Depending on the foaming agent, the appropriate pH Writing – review & editing. Tuan A. Le: Writing – review & editing.
value of the base mix help maintain the viscosity of foam slurry and
strengthen the liquid film, resulting in an even pore distribution with Declaration of competing interest
a narrow size. Moreover, the use of magnetised water in lieu of
regular tap water in the foaming process can strengthen a hydrogen The authors declare that they have no known competing financial
bond with foaming surfactant molecules due to the intermolecular interests or personal relationships that could have appeared to influence
forces. However, this approach is more suitable for synthetic than the work reported in this paper.
natural foaming agents. Surface coating through impregnation into
lithium silicate, aerogel solution or composite emulsion enhances the Data availability
durability of foam concrete. Higher foam stabilisation can also be
regulated by applying microwave and ultrasonic pre-treatment. No data was used for the research described in the article.
7. The combination of existing strategies achieves synergetic effects to
maximise the performance of foam concrete for various semi- Acknowledgement
structural building applications, especially the emerging 3DP foam
concrete technology (Section 5). Although numerous studies have The authors gratefully acknowledge the financial support of the CRC-
been done in this area, several challenges (Section 6) still need to be P projects on recycling construction waste for manufacturing sustain
addressed through future research and development. able materials, funded by the Department of Industry, Innovation and
Science, Australia.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2022.133939.
Figures in the Publishers of Published Journal Figure number in Figure title in published journal Reference paper in published journal
drafted Journal the published
manuscript journal
1 John Wiley Structural Concrete Fig. 2 (a) Lightweightconcrete (Gartmann house) Elshahawi, M., A. Hückler, and M. Schlaich,
and Sons (Permission is not Schweiz 2003 density; 1100 kg/m3, thermal Infra lightweight concrete: A decade of
required for open conductivity; 0.32 W m− 1K− 1,strength; 12.9 investigation (a review). Structural Concrete,
access article) MPa (ACI Committee 213R-03, 2003). (b) 2021. 22(S1): p. E152-E168.
Infralightweight concrete (Schlaichhouse)
Berlin 2007 density; 760 kg/m3, thermal
conductivity;0.18 W m− 1K− 1, strength; 7.4
MPa (Nodehi, 2021). (c) Lightweight concrete
(houseH36) Stuttgart 2012 density;1000
kg/m3, thermal conductivity;0.23 W
m− 1K− 1, strength; 10.9 MPa (ACI Committee
213R-03, 2003). (d) Infra lightweight concrete
(Pavilion) TU Eindhoven 2015 density; 780
kg/m3, thermal conductivity; 0.13 W
m− 1K− 1,strength; 10 MPa (ACI Committee
213R-03, 2003)
2 * Self-made figure
3 * Self-made figure
4 Elsevier Construction and Fig. 12 (a) 2D schematics of the forces acted on single Li, G. et al., The influence of wet ground fly ash
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Francis Concrete Structures components of cement paste on foam for foamed cement paste, in Tailor made
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required for open Stoelhorst, Editors. 2008, Taylor & Francis
access article) Group: London. p. 457-60.
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7a MDPI Materials Fig. 7 Changes in zeta potential of cement particles Liu, Q., Z. Chen, and Y. Yang, Study of the Air-
(Permission is not with increasing surfactant concentration. Entraining Behavior Based on the Interactions
required for open Arrows signify the concentration of each between Cement Particles and Selected Cationic,
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